Quantum Monte Carlo description of correlated electrons in two-dimensional FeSe
An interesting question in physics is how the correlation energy of atoms evolves upon forming a solid. Here, we address this problem for a specific case of double-layer FeSe. We used many-body wavefunction-based quantum Monte Carlo (QMC) techniques to compute the correlation energies of double-layer FeSe with different geometrical configurations and compared them with those of isolated Fe and Se atoms. Variational and diffusion QMC calculations were carried out with Slater Jastrow trial wavefunctions employing two alternative forms for the homogeneous two-body pair correlation term. The ground-state energy was obtained in the thermodynamic limit using two types of trial wave functions of JDFT, in which only the Jastrow factor is optimized while the Slater determinant is derived from the local density approximation, and JSD, where both the Jastrow factor and the Slater determinant are optimized simultaneously. Our results indicate that the correlation energy of double layer FeSe at the thermodynamic limit is mainly determined by the atomic contributions, with the bonding between atoms playing a comparatively minor role in it. After optimizing the interlayer separation of double-layer FeSe under tensile strain, we analyze the correlation energy as a function of strain and separation. We found that with increasing tensile stretch and interlayer spacing, the correlation energy of double-layer FeSe stochastically approaches that of its constituent atomic fragments.